Synchrotron-Based X-Ray Fluorescence Microscopy Reveals Accumulation Of Polymyxins In Single Human Alveolar Epithelial Cells

Intravenous administration of the last-line polymyxins results in poor drug exposure in the lungs and potential nephrotoxicity, whereas inhalation therapy offers better pharmacokinetics/pharmacodynamics for pulmonary infections by delivering the antibiotic directly to the infection site. However, polymyxin inhalation therapy has not been optimized, and adverse effects can occur. This study aimed to quantitatively determine the intracellular accumulation and distribution of polymyxins in single human alveolar epithelial A549 cells. Cells were treated with the iodine-labeled polymyxin probe FADDI-096 (5.0 and 10.0 mu M) for 1, 4, and 24 h. Concentrations of FADDI-096 in single A549 cells were determined by synchrotron-based X-ray fluorescence microscopy. Concentration- and time-dependent accumulation of FADDI-096 within A549 cells was observed. The intracellular concentrations (mean +/- standard error of the mean [SEM], n >= 189) of FADDI-096 were 1.58 +/- 0.11, 2.25 +/- 0.10, and 2.46 +/- 0.07 mM following 1, 4, and 24 h of treatment at 10 mu M, respectively. The corresponding intracellular concentrations following the treatment at 5 mu M were 0.05 +/- 0.01, 0.24 +/- 0.04, and 0.25 +/- 0.02 mM (n >= 189). Over 24 h, FADDI-096 was mainly localized throughout the cytoplasm and nuclear region. The intracellular zinc concentration increased in a concentration- and time-dependent manner. This is the first study to quantitatively map the accumulation of polymyxins in human alveolar epithelial cells, and it provides crucial insights for deciphering the mechanisms of their pulmonary toxicity. Importantly, our results may shed light on the optimization of inhaled polymyxins in patients and the development of new-generation, safer polymyxins.